Partial recovery of the stimulatory oestrogen feedback action on LH release during late lactation in the pig.

The functioning of the stimulatory oestrogen feedback mechanism during lactational anoestrus of the sow was evaluated by determining the LH response to a single i.m. injection of oestradiol benzoate. Oestradiol-17 beta and progesterone levels were also measured following treatment and following weaning. On Day 5 of lactation there was no LH response to oestradiol benzoate but on Day 35 of lactation a small but significant (P less than or equal to 0.05) increase in plasma LH occurred, and this was followed by a rise in plasma progesterone in 4 out of 9 animals 8 or 13 days later. It is concluded that blockade of the stimulatory oestrogen feedback mechanism is one cause of lactational anoestrus in the sow.